Currently, the glass manufacturing industry is under considerable pressure to reduce the amount of pollutants which are emitted into the atmosphere as a result of the glass melting process. One such particular family of pollutants is nitrogen oxide compounds (commonly referred to as NOX). In this regard, oxygen/fuel (oxy-fuel) burners have been employed in place of the more conventional air fuel burners which historically were employed in glass making furnaces. Air consists of about 78 percent nitrogen, while the "oxygen" gas utilized in oxy-fuel burners typically contains about 0.1-6 percent nitrogen and 0.1-4 percent argon, the remainder being oxygen (typically 90-99.8 percent). By employing oxy-fuel burners, the amount of nitrogen (NOX) emitted by such glass furnaces into the atmosphere is reduced. Unfortunately, in some glass batches it has still been desirable to employ some nitrogen containing materials in the batch. For example, niter is commonly employed because of its ability to enhance the oxidizing ability of the glass batch. Niter, which can be, for example, in the form of potassium nitrate, sodium nitrate, or barium nitrate, releases oxygen into the glass batch, thereby oxidizing the batch directly (as opposed to providing a more oxidizing atmosphere above the glass).
One family of glasses for which nitrogen containing compounds continue to be a particular problem are lead containing glasses. Lead-containing glasses effectively shield high-energy radiation and are therefore used commercially for radiation windows, fluorescent lamp envelopes, and television bulbs. During the manufacture of lead-containing glass, if the atmosphere inside the glass melting furnace is not sufficiently oxidizing, the lead oxide has a tendency to reduce and precipitate as a liquid metal, resulting in a deficiency of lead oxide in the glass. Such precipitation of the lead could have extremely deleterious consequences, as the lead oxide is utilized to protect consumers from x-rays produced inside the television.
As a consequence, in the television tube and other lead glass making industries, relatively large quantities of niter have commonly been added as a glass batch material to provide an extremely oxidizing atmosphere inside the batch blanket. The niter releases oxygen into the glass melt, thereby preventing reduction of the lead oxide. Unfortunately, the niter also contains nitrogen, and consequently results in increased NOX which are exhausted from the glass tank furnace.
Historically, lead glasses were melted using air/fuel burners, and niter was added in relatively large quantities to prevent reduction of the lead oxide in the melt to lead metal. Although oxy-fuel burners have since been employed in place of the air/fuel burners in such glass melting processes, the lead glasses have still utilized relatively large quantities of niter compounds in the glass batch to prevent precipitation of lead oxide. For example, in some television tube manufacturing furnaces which do not employ the methods of the present invention (but do employ oxy-fuel burners), 75 to 80 percent of the total NOX emitted from the process is due to the use of niter as a batch material.
It would therefore be desirable to be able to manufacture glasses which require strongly oxidizing atmospheres, particularly lead containing glasses, without the need for having to use large quantities of nitrogen containing materials in the batch.